Lightning and Switching Overvoltages With easy examples.

Overvoltages are voltage spikes above the normal operating voltage. The two main types are lightning overvoltages, from strikes to lines or towers, and switching overvoltages, from operating breakers, switching capacitor banks or clearing faults. Both last only microseconds to milliseconds, but their peaks can be several times the normal voltage.

They matter because insulation is designed for a specific maximum stress. A 220 kV line rated for a 950 kV impulse can be punctured by a severe 1500 kV lightning surge. Every major blackout investigation asks the same question: did an overvoltage exceed the insulation level?

Key Concepts

• Lightning surge: a strike injects huge current into the line or tower; the voltage wave travels along conductors in both directions with a very steep front, rising in about 1.2 microseconds.
• Back flashover: when lightning hits a tower, the tower voltage rises so high it flashes over backwards across the insulator string to the conductor, though the strike never touched the wire.
• Switching surge: interrupting inductive current or energising a long line causes voltage oscillations. In current chopping, a breaker interrupts a small inductive current suddenly and the trapped magnetic energy becomes a high voltage spike.
• Restrike: when interrupting capacitive current, the recovering voltage can exceed the contact gap's recovering strength, re-igniting the arc and multiplying the voltage in steps.
• Travelling wave: a surge moves at nearly the speed of light, reflecting at every junction or open end. Reflection can nearly double the voltage at an open line end.

Formulas

• Wave speed: v = 1 / √(L×C), with L and C per unit length. For overhead lines this is about 300 metres per microsecond.
• Voltage from injected current: V = I × Z / 2, where Z is the line's surge impedance (about 400 Ω). The current splits into waves in both directions, hence the division by 2.
• Travel time: t = length / v, the time for a surge to travel from the strike point to the substation.

Worked Example

A 20 kA lightning stroke hits an overhead line directly. Injected voltage: V = 20,000 × 400 / 2 = 4,000,000 V, or 4000 kV, splitting into two waves racing both ways. If the nearest substation is 30 km away, travel time t = 30,000 / 300 = 100 microseconds. The surge arrives in just 100 µs — far too fast for any breaker, which is why nanosecond-fast surge arresters are the defence.

Common Misconceptions

• "Lightning must hit the wire to damage equipment." Most damage comes from tower strikes causing back flashover, or from nearby ground strikes inducing voltage. Direct wire hits are only a fraction of cases.
• "Switching surges are always smaller than lightning." Above 300 kV, switching surges decide the insulation design, because their longer duration stresses insulation more than a brief lightning impulse of the same peak.
• "A surge disappears at the substation." Waves reflect and ring between discontinuities for many microseconds; voltage at an open breaker can nearly double the incoming wave.

Real-World Applications

• Shield wires: an earthed wire above the phase conductors intercepts most strokes before they reach live wires.
• Line-entrance arresters: station-class arresters clamp incoming travelling waves before they reach transformers.
• Controlled switching: breakers close at voltage zero (point-on-wave switching) to minimise surges when energising lines and capacitor banks.

Lightning sends kilovolt waves racing along lines at 300 metres per microsecond, while switching operations create oscillatory surges that dominate EHV design. Shield wires, fast arresters and careful switching tame both — and V = I×Z/2 shows why even a moderate 20 kA stroke creates thousands of kilovolts.